Microstrip antennas (or patch antennas) provide low-profile antenna configurations for applications that require small size and weight. Such antennas are also desirable when there is a need to conform to the shape of the supporting structure, both planar and nonplanar, such as for an aircraft's aerodynamic profile. These antennas are simple and inexpensive to manufacture using printed-circuit technology, wherein metallic patches (or patch radiators) are typically photoetched onto a dielectric substrate.
The conventional wisdom regarding microwave patch antennas is that the patches radiate from their edges. More specifically, when the elements of a patch antenna array are excited in common mode (i.e., with equal voltages), the fields that are generated are primarily confined to the dielectric space under each surface element, except for the fringing fields at the edges of the elements. The commonly held view of the mechanism of radiation by patch antennas is that it is the fringing fields at the edges that radiate into the air. Indeed, various models and theoretical analyses have been developed to explain this radiation mechanism, such as the slot radiation model (see, e.g., R. E. Munson, “Conformal microstrip antennas and microstrip phase arrays,” IEEE Trans. Antennas Propagat., vol. 22, pp 74-78. January, 1974) or the cavity model (see, e.g., Thouroude et al, “CAD-oriented cavity model for rectangular patches,” Elect. Lett., vol. 26. pp. 842-844, June 1990). Both the slot and cavity models assume radiation comes only from the edges. Other models known to those skilled in the art, including, for example, conformal mapping, moment methods, and Green's functions, have been developed, which implicitly include fields that are not at the edges. However, these methods offer limited insight into the radiation mechanism.
FIG. 1 illustrates a typical patch antenna array 10 that comprises small conducting surfaces 18 separated from a large parallel ground plane 14 by a dielectric substrate 16. When the same real or complex (real and imaginary or amplitude and phase) RF voltage V0 is applied to each surface 18, an electric field pattern 15 is set up in the dielectric, essentially acting as a capacitor but with a relatively weak fringing fields 12 at the edges (for clarity, fields 12 are not shown continuing into the substrate). The roughly uniform fields 15 under the surface are fairly well shielded from the outside space, but the fringing field at the edges can act as radiating elements. To take advantage of the edge radiators, it may be necessary to excite the capacitive structure in a higher-order mode and using off-center feeds, to avoid mutual cancellation of the radiation from different edges.
Microstrip patch antennas commonly exhibit disadvantageous operational characteristics such as low efficiency, low power, narrow bandwidth, and poor scanning performance. Further, patch antennas are typically excited in an asymmetric manner to generate high-order modes of the dielectric substrate, which adds to the complexity of the electrical feed circuitry.
A natural phenomenon referred to as “mutual coupling” occurs when the patches of an antenna array are subjected to differential-mode excitation (e.g., different voltage amplitudes and phases). In particular, when the applied voltages at two or more patches are different, fields will be set up not only within the substrate directly under each patch, but also in the air space above the patches, emanating from one patch and ending on another.
Conventionally, designers of patch antennas ignore or attempt to reduce the effects of mutual coupling. However, it would be highly beneficial to develop a framework for differential-mode excitation of an antenna array that would exploit the mutual coupling between patches to provide efficient radiation from the exposed top surfaces of antenna patches to, thereby, overcome the above noted deficiencies and disadvantages of conventional patch antenna schemes.